I rememeber being told in seminary that a preacher ought to read a new book on preaching every year. At the time, I thought that there can't be that many books on preaching. But browsing the internet - there are so many titles published on the subject - many more than I have listed below. Here are some titles that I have found either useful or seem pratical and interesting. For preachers on a budget, check out the prices on used copies of these on Amazon - many cost with postage less than a fast food meal!
Design for Preaching
by H. Grady Davis. By the "Wittgenstein of homiletics," this 1958 book continues to be one of the best books on preaching. No matter how many years you have been preaching, your preaching will improve as you read and think about what Henry Davis has to say.
and a couple more. . .
Homiletic Moves and Structures
by David G. Buttrick. His homiletic is geared toward constructing sermons that people will actually remember after they leave worship. He also spends half of the book on the art of interpreting the biblical text with regards to preaching, arguing that different literary forms demand different types of sermons.